About 800,000 people die each year from malaria. While the currently available vector control tools-mainly bed nets and indoor insecticide spraying-are substantially reducing malaria cases and deaths, this public health success is fragile due to the threat of resistance. Moreover, even without resistance, existing tools are insufficient to interrupt malaria transmission where it is high and stable, as in many parts of Africa. The development of new vector-targeted tools is imperative, but depends on an understanding of mosquito ecology and behavior that is currently lacking. Here we study saltwater tolerance, a trait found in numerous important malaria vectors that plays a key role in determining habitat use and ecological distribution of mosquitoes, and thus their contribution to malaria transmission. Our objective is to dissect the genetic basis of saltwater tolerance in the African malaria vector An. merus, a close relative of the primary vector An. gambiae. We will dissect saltwater tolerance in An. merus through two complementary specific aims: 1. Genetically map QTLs that contribute to salinity tolerance in An. merus Using multiple colonies of An. merus and An. gambiae, we have shown that An. merus can be distinguished from An. gambiae and their F1 hybrids by survival in 50% seawater. We will apply a novel Illumina-based genotyping approach to map recombination breakpoints in individual backcross progeny that do/do not survive exposure to 50% seawater, to localize QTL regions that control salinity tolerance. 2. Identify differential gene expression associated with development in fresh vs. saltwater We have shown that the ability of An. merus to survive in 50% seawater is dependent upon developmental timing of exposure, and that the localization of an ion transporting protein (Na/K ATPase) in the rectum differs in fresh vs. saltwater-reared larvae. We hypothesize that these observations are due to differential expression triggered by exposure to saltwater. We will test this hypothesis and identify candidate genes by comparing global gene expression between fresh vs. saltwater-reared larvae. At the end of the two-year project, combined evidence from QTL mapping and differential gene expression will lead us to candidate genes and/or candidate gene regions that contribute to salinity tolerance in the malaria vector An. merus. Unlike other complex ecological, behavioral and life history traits of epidemiological importance that are probably polygenic, saltwater tolerance is relatively tractable, likely governed by a few major loc with large effects, and simple to assay. The ability to dissect the genetic basis of this adaptive trait using next generation genomic tools lays the groundwork for future efforts to understand the mechanisms by which these vector mosquitoes adapt to a heterogeneous and changing environment, opening up new avenues of vector control.